The present invention relates to preventing beat interference between receiving devices and, more particularly, to a receiving system which prevents beat interference between proximately situated receivers receiving a common radio signal.
An example of a receiver which receives radio frequency signals from a transmitter and converts them into an audio signal suitable for audio reproduction by, for example, a speaker, is shown in FIG. 3, wherein the radio signal Si is received via an antenna 2 of receiver 1. The radio signal Si is filtered by band-pass filter 3, amplified by high frequency amplifier 4 and filtered again by band-pass filter 5. A mixer 6 generates a fixed band signal by mixing the filtered signal with a lower frequency local oscillation signal S2 provided by a local oscillator 7. The fixed band signal S3 is further filtered by band-pass filter 8 and amplified by high-frequency amplifier 9. This signal is, then, mixed in mixer 10 with another local oscillation signal S4 generated by local oscillator 7 to produce an intermediate frequency signal of an intermediate frequency. An IF amplifier 11 amplifies this intermediate frequency signal and a detector 12 detects the amplified IF signal to generate the audio signal S6 suitable for audio reproduction.
An example of local oscillator 7 is shown in FIG. 4, wherein an internal oscillator 13 generates an internal clock signal S7 which is frequency divided by frequency dividers 14, 15 and coupled to respective phase locked loop (PLL) circuits 16, 17. It will be appreciated that the internal clock signal S7 may be supplied additionally to a control circuit (not shown) of a CPU or other processor as a reference clock. Each of the two PLL circuits in FIG. 4 have the same construction in this example and each includes a phase comparator 18 (21) which generates a phase difference signal indicating a difference in phase between the internal clock signal and a local oscillation signal generated by a voltage controlled oscillator (VCO) 20 (23) and fed back through a frequency divider 21 (24). A low pass filter 19 (22) band-passes this phase difference signal to VCO 20 (23) which generates the local oscillation signal S2 (S4).
Problematically, if two supposedly identical receivers include local oscillator 7, the local oscillation signal generated by the local oscillator in, for example, receiver 1A of FIG. 5 and interferes with the local oscillation signal generated by the local oscillator of proximately situated receiver 1B, such as when receivers are used to provide useful information during a xe2x80x9cwalking tourxe2x80x9d. This situation arises, for example, when the conductive housing of the receiver acts as an antenna and transmits the local oscillation signal. Attempts to shield the housing, or to insert an isolator between the receivers, or to split the antenna signal between the receivers 1A, 1B (FIG. 6) has been found inadequate to prevent the interference because some leakage of the local oscillation signal always occurs.
The problem described above is particularly noticeable when two local oscillation signals from different receivers have slightly different frequencies, which may be due to the physical characteristics of each receiver or the difficulty arising out of adhering to very close manufacturing tolerances. As shown by the frequency-domain graph of FIG. 7, for example, two local oscillation signals associated with receivers 1A and 1B may be set at the slightly different frequencies of 850 and 851 MHz. These local oscillation signals (1A, 1B) combine to create high frequency interference signals (a, b) near the frequencies of 852 and 849 MHz, respectively. When a third receiver (1C) has a local oscillation signal at the frequency of 852 MHz, a difference (d) between this frequency and the frequency of the nearby interference signal (a) creates interference which manifests in the reproduced audio sound as a beat.
This xe2x80x9cbeatxe2x80x9d interference arises conspicuously when receivers are situated proximate enough to receive the interfering signals. An example of this proximate situation is a tour group of people shown in FIG. 8A touring through a museum and carrying wireless receiver head-sets which receive a common radio signal of audio information about a viewed museum piece. Since these local receivers are tuned to receive the common radio signal, the generated local oscillation signals of slightly different frequencies are combined to generate mutual interference which is picked up by the other receivers, thus, causing beat interference. It will be appreciated that shielding the receivers by inserting an isolator or by splitting the antenna between a plurality of receivers in such a tour group is impractical because such measures would restrict the user""s movement.
Since beat interference arises when local oscillation signals exhibit different frequencies, it is theoretically possible to prevent beat interference by using receiver components of precisely the same electronic characteristics for all receivers because this would cause the local oscillation signals in each receiver to have the same frequency. However, it is quite difficult and expensive to manufacture a plurality of receivers with the same electronic characteristics. Typically, an internal oscillator has a precision of xc2x15 [ppm], meaning that is such an oscillator tuned to generate 850 MHz will, in fact, generate 850 MHzxc2x14.25 KHz. This problem is compounded when there are more than two receivers which need to be precisely manufactured, such as when a large group of users tour a museum with many receiver head-sets. Problematically, beat interference is audibly disturbing to such individuals, seriously diminishing the acceptance of such audio signal reproduction.
It is an object, therefore, to provide an apparatus and method for preventing beat interference in the aforementioned environment.
It is a further object to provide an apparatus and method for preventing beat interference arising from mutual interference of a plurality of receivers.
It is an even further object to provide an apparatus and method for preventing beat interference caused by a difference in local oscillation signal frequencies among a plurality of proximate receivers.
In accordance with the above objectives, the present invention provides an apparatus and method for preventing beat interference among receivers by selecting either an external clock signal that is transmitted to all receivers or, if no external clock signal exists, selecting an internal clock signal generated internally by each receiver. With this invention, each receiver can receive the same external clock signal as other receivers and the local oscillation signal for that receiver may be set precisely to the local signal oscillation frequency thereby preventing beat interference from arising.
In addition, the present invention also provides a receiver system formed of a plurality of receivers coupled in parallel to form a relay. Each of the receivers selects the external clock signal transmitted thereto, and in the absence of the external clock signal selecting the internal clock signal, thereby setting each of the respective local oscillation signals to the same frequency to prevent beat interference arising from a difference of local oscillation frequencies.